The present invention relates to an operating circuit, and more particularly, to a squelch circuit that utilizes a clamping device to increase the operating speed of the squelch circuit.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a prior art squelch circuit 100 of. The squelch circuit 100 comprises a first input circuit 104, a first comparator 108, an inverter 110, a second input circuit 112, a second comparator 116, an inverter 118, and a de-glitch circuit 120. The squelch circuit 100 is utilized for detecting a differential voltage level of the eye diagram of a differential signal (i.e., VIN+, VIN− as shown in FIG. 1) inputted to the level shift circuit 102 and generating a detected output SQOUT. The differential signal VIN+, VIN− is transmitted into two signal paths, the first signal path is comprises of the first input circuit 104, the first comparator 108, and the inverter 110, while the second signal path is comprises of the second input circuit 112, the second comparator 116, and the inverter 118, wherein the differential signal VIN+, VIN− that transmitted into the first path is inversed to the differential signal VIN+, VIN− that transmitted into the second path.
According to the specification of the USB (Universal Serial Bus), the minimum voltage level that required by the USB receiver is 100 mV, thus the squelch threshold is set about 125 mV normally. Therefore, the first signal path is responsible for detecting if the voltage difference of VIN+−VIN− is smaller than −125 mV, and the second signal path is responsible for detecting if the voltage difference of VIN+−VIN− is larger than 125 mV. If the voltage difference of VIN+−VIN− is smaller than −125 mV, then an output OUT1 of the first comparator 108 is a high voltage signal; if the voltage difference of VIN+−VIN− is larger than 125 mV, then an output OUT2 of the second comparator 108 is a low voltage signal. Thus, after the de-glitch circuit 120, if the voltage difference of each eye diagram of the differential signal VIN+, VIN− is larger than the squelch threshold 125 mV, the detected output SQOUT will be always a high voltage; if the voltage difference of a specific eye diagram of the differential signal VIN+, VIN− is lower than 125 mV, the de-glitch circuit 120 will output a low voltage that corresponding to the specific eye diagram of the differential signal VIN+, VIN−. However, due to the parasitic resistor and capacitor at the output nodes of the first stage one circuit 104 and the second input circuit 112, the signal swing at the output nodes of the first input circuit 104 and the second input circuit 112 may not reach to the designed target within a specific time interval. On the other hand, the operating speed of the first input circuit 104 and the second input circuit 112 may limit the detecting speed of the squelch circuit 100. Therefore, increasing the operating speed of the first input circuit 104 and the second input circuit 112 is becoming a critical technique in the field of high speed USB devices.